Message from the President
In ICUMSA News No 24 (May 1995) I spoke of the need to replace old classical methods with cost effective, instrumental methods. Many of our readership would be aware that my own laboratory – CSR Central Laboratory, carries out analysis of the Queensland Industry’s raw sugars for the purpose of payment and quality monitoring. Because we face cost pressures, we are seeking to replace some of the industry’s methods with more cost-effective ones. To this end we are considering replacing sulphated ash with conductivity ash and water content by loss on drying with a near Infrared (NIR) spectroscopic method. In the latter case, the NIR method would employ the loss on drying method for the calibration set.
The method, that we would dearly like to replace is the determination of particle size (mean aperture and coefficient of variation) in raw sugar. This is the method our analysts like the least because of the extensive preparative procedures required before the sugar is presented for sieve analysis. We have explored the use of particle size analysers and have experienced difficulties getting results comparable to our present method and with the repeatability of the instrumental method. We also have misgivings about using a sample of just a few grams to represent 2000 tonne quantities of raw sugar.
I mention our experiences to illustrate some of the issues one faces in seeking new cost-effective methods. We would certainly welcome discussing these matters with others sharing our experiences or just interested in the challenge of developing new methods. There may be value in discussing matters like this in future issues of ICUMSA News
White Sugar
Progress Report on General Subject 2: White Sugar
Geoff Parkin, Referee, General Subject 2
A work programme has been agreed in principle with 27 Associate Referees of this Subject and with Referees of related Subjects. At this time I am confirming with individuals that they are prepared to participate, first of all in preliminary studies to assess their accuracy and repeatability in their chosen methods, and later in full-scale collaborative studies, conducted according to the IUPAC Protocol.
Methods which everyone consulted has agreed to be tested, and which accord with the Recommendations adopted for General Subject 2 (see ICUMSA News No. 22, September 1994), are listed in sequence of those Recommendations:
Recommendation 2: Method GS 2/3-5 “The Knight and Allen method for reducing sugars in white sugar”.
Twelve potential participants will shortly be asked to comment on the revision of this method which have been made by Mr. R.W. Plews of the British National Committee. The finally revised method will then be tested in conjunction with tests on the same samples of white sugar using:
a) The Berlin Institute method (Recommendation 4) to be organised by Dr. L.B. Jørgensen (Referee, Subject 15) as a full-scale collaborative study.
b) A capillary GC method to be used by Associate Referee H. Hoebregs.
c) An HPIC method to be used by Mr. K.J. Schäffler (Referee, Subject 8).
Recommendation 5: Method GS 2/1/3-15 “Moisture by Loss on Drying”
It has been agreed with Prof. G. Vaccari (Referee, Subject 5) and Associate Referee Paul Caulkins that it is desirable to test more up to date methods. Prof. Vaccari is already putting in hand tests on white sugars using a Karl Fischer method devised by Mr. M.K. Faviell and Mr. Caulkins will conduct collaborative studies with instrumental IR driers. My own laboratory will also test Method GS2/1/3-15 on the same circulated samples.
Recommendation 6: Method GS 2/3-19 “Insoluble matter”.
Fourteen potential participants will shortly receive a slightly modified Method GS 2/3-19 for their comments. Early in 1996 preliminary tests of accuracy and repeatability will be conducted.
Recommendation 9: GFAAS for As and Cu.
The names of five potential participants have been submitted to Dr. Margaret Clarke (US National Committee) for inclusion in the proposed ILSI/AOAC International tests.
Recommendation 11: Method GS 2-13 “Reflectance”.
This method will shortly be circulated to eleven potential participants for comments before preliminary testing in 1996.
While the above programme seems to me to be comprehensive, I would welcome any comments at all, particularly at this early stage. If anyone believes other methods should be included, please make them known to me. I will then consult with all those involved in the work planned at this time.
Colour in white sugar
David Hawkins has sent us the following letter:
Sir
CSR Refined Sugars have recently experienced difficulty in getting satisfactory agreement between the ICUMSA Method for colour of white sugar and the method used by a number of customers. Upon investigation it turned out, that the 0.45 mm membrane filtration of the solution for the ICUMSA method removed significant colour.
Colour of white sugar is an important quality consideration for the carbonated beverage industry with standards in many countries related to the RBU Method. In this method 720 nm absorbance is used to correct for the effect of turbidity. Until recently we believed the ICUMSA Method (0.45 mm filtration) gave essentially the same results as the RBU Method at the same pH.
Recent investigations have demonstrated that for sugar solutions with zero or very low 720 nm absorbance, it is possible for the ICUMSA colours to be as much as 10 units lower than the RBU colour, for colours of the order of 30 units. It is clear, that visible colour is being filtered out, at times, by the ICUMSA Method. This is commercially unacceptable.
It seems to us that while the purpose of membrane filtration is to remove turbidity, it can also remove significant colour from commercial sugars. We are aware of the paper by J.B. Kuntz (Sugar y Azucar, Feb. 1993, pp.30–35) which also points out, that different membrane types give different results suggesting selective removal of colour. We therefore ask, what is proposed to resolve this unsatisfactory state of affairs. We cannot support the ICUMSA Method as a basis for trade in its present form, as from a customer’s point of view, it can grossly underestimate the colour effect on their process.
David Hawkins
We have asked the Referee for subject 7: Colour, Turbidity and Reflectance measurements, Richard Riffer, to comment on this letter.
Referees Comments
by Richard Riffer
Light scattering is always present in sugar solutions, with scattering particles covering a range from smaller than 0.1 mm to larger than 8 mm. At the high end, the sedimentation rate is appreciable, so at some point it is no longer appropriate to describe larger particles as suspended solids. For intermediate particle sizes, that settle on the order of minutes, the contribution to turbidity is of course unstable with time.
Although particles smaller than 0.45 mm are sometimes defined as colloidal, it is unrealistic to expect that any such arbitrary cutoff point has a basis in nature. The choice is in fact purely a convenience, tighter filtrations tend to be impractically slow or to require extreme dilution, and looser ones are apt to be less effectual.
The filtration step is known to be an important contributor to poor precision in colour measurements. The difficulty posed by the arbitrary nature of the filtration tightness mentioned above is exacerbated by variability in pore diameter and size distribution in the arbitrary nature of the ostensibly 0.45 mm commercial filters. This lack of pore uniformity together with the range of hydrophilicity of available membrane materials, can effect a partitioning of colorants by both polarity and size during filtration. Low molecular weight colorants present at early processing stages would be expected to be more polar than their higher molecular weight reaction product counterparts, because carboxyl, carbonyl and phenolic functions are expended during polymerization and crosslinking, diluting their contribution to the polarity of the product. In addition, dehydration is an important caramelization mechanism, likewise resulting in diminished polarity. Some “high molecular weight colorants” are not instrinically polymeric but instead low molecular weight material bonded to colourless polymeric species.
Joseph B. Kuntz (1993A, 1993B), in his study of the effect of membrane composition on color and turbidity measurements, generally found greater 420 nm absorbance using hydrophilic membranes of the cellulose ester type (although one hydrophobic membrane, a vinyl acrylic copolymer appeared to behave anomalously). The wetted hydrophilic surfaces are evidently cushioned with adsorbed water molecules and hence less accessible for intimate contact with hydrated colorant molecules, which typically have both hydrophilic and hydrophobic zones. Unfortunately, the contribution of microfilter pore size distribution to Dr. Kuntz’s findings is unknown.
ICUMSA News editor, Rud Frik Madsen (1994), who has had experience in microfilter manufacture, has remarked on the lack of standard method among manufacturers to characterize pore size.
The reported value can be an average or a figure close to the maximum. Moreover, the pore size distribution for different suppliers can vary depending on the manufacturing method. Microfilters from some suppliers can contain a large number of pores much smaller than the nominal size. Such variability certainly has the potential to affect the optical properties of the filtrate.
Among my Subject 7 ICUMSA and U.S. National Committee Associate Referees there is a broad consensus that the proper way to study turbidity is by measurement of 90° scatter. However, we have thus far been unable to identify a sufficient number of potential participating laboratories to develop a collaborative test that will meet statistical requirements. We would like to use the identical nephelometer model to avoid the introduction of a variable of instrument design, to which turbidimetry is far more sensitive than is colorimetry. Additional study of 90° scatter for turbidity measurement was Recommendation 2 at last year’s Havana session. We would also like to study further the filtration step, focusing particularly on the effect of membrane composition. I would like to take this occasion to invite interested parties to contact me in California at fax (+1) (510)787-2916.
Measurement of the turbidity of sugar solutions at 90° is not necessarily the best choice, because sugar solutions scatter much more strongly at forward angles, but instrumentation for other options is far less likely to be widely available for collaborative work. Because of the broad size range of scattering particles in sugar solutions and the fact that scattering from such samples is not of the Rayleigh type, theoretical Mie treatments are likely to fail.
References
Kuntz, J. B. (1993 A): Sugar y Azucar 88 (2), 1993, 30–35
Kuntz, J.B. (1993 B): ICUMSA News No. 19, 1993: Zuckerind. 118, 710– 712; Int. Sugar J. 95, 369–70
Madsen, R.F. (1994): Private communication
Recertification of Quartz Plates
Zoltan Hegedus has sent us the following letter:
Sir
From time to time the Australian National Measurement laboratory receives enquiries about recertification of quartz plates for use in the sugar industry. We used to undertake this work but because of scarce resources and the need to refurbish outdated equipment we no longer offer this service.
In coming to this decision, we questioned the need for recertification because it is our view that once properly calibrated, we cannot envisage circumstances that would cause a quartz plate value to change.
We can appreciate circumstances where quartz plates can become damaged, for instance, through efforts to clean them. However, if a quartz plate is damaged, this can be readily determined by simple inspection without measuring optical rotation.
Referees Comments
by Albert Emmerich, Referee Subject 4: Polarimetry and Quartz Plates
I fully agree with Dr. Hegedus’ opinion, that it is unnecessary to recalibrate quartz plates which have been properly calibrated by an institution which is able to check all aspects laid down by ICUMSA. That means not only the rotation value for at least one exactly defined wavelength must be determined with the necessary accuracy, but also the other properties, like flatness and plane parallelism of the surfaces, absence of non-homogeneities and the exact position of the optical axis. If such a plate is handled with the necessary care the rotation will not change.
But what about the inspection and calibration of new plates? If the official Australian laboratories are not willing to take over this task in the future, there will be only one institution all over the world which is able to do the job, viz. the PTB in Braunschweig. This is – to my opinion – no satisfactory state. The PTB is, at the moment, surely bound to calibrate plates within the European Community and perhaps for other European states. But I doubt whether in the long run they will be able and willing to take over this duty for all other countries.
Editor: Rud Frik Madsen, Strandpromenaden 38, DK-4900 Nakskov, Denmark – Tel: +45 53921675; Fax: +45 54951675